Gram-positive microorganisms, such as members of the group Bacillus, have been used for large-scale industrial fermentation due, in part, to their ability to secrete their fermentation products into the culture media. In gram-positive bacteria, secreted proteins are exported across a cell membrane and a cell wall, and then are subsequently released into the external media usually maintaining their native conformation.
Various gram-positive microorganisms are known to secrete extracellular and/or intracellular protease at some stage in their life cycles. Many proteases are produced in large quantities for industrial purposes. A negative aspect of the presence of proteases in gram-positive organisms is their contribution to the overall degradation of secreted heterologous or foreign proteins.
The classification of proteases found in microorganisms is based on their catalytic mechanism which results in four groups: the serine proteases; metallo-proteases; cysteine proteases; and aspartic proteases. These categories can be distinguished by their sensitivity to various inhibitors. For example, the serine proteases are inhibited by phenylmethylsulfonylfluoride (PMSF) and diisopropylfluorophosphate (DIFP); the metallo-proteases by chelating agents; the cysteine enzymes by iodoacetamide and heavy metals and the aspartic proteases by pepstatin. The serine proteases have alkaline pH optima, the metalloproteases are optimally active around neutrality, and the cysteine and aspartic enzymes have acidic pH optima (Biotechnology Handbooks, Bacillus. vol. 2, edited by Harwood, 1989 Plenum Press, New York).
Metallo-proteases form the most diverse of the catalytic types of proteases. About half of the families comprise enzymes containing the His-Glu-Xaa-Xaa-His (or HEXXH) motif which has been shown by X-ray crystallography to form part of the site for binding of the metal (normally zinc) atom. In one family of metalloproteases, a glutamic acid residue completes the metal-binding site, HEXXH+E. This family contains the most well characterized of the metallo-proteases, thermolysin. The three dimensional structure of thermolysin shows that, in the HEXXH motif, the His residues are zinc ligands and the Glu residue has a catalytic function. (Methods in Enzymology, vol. 248, Academic Press, Inc. 1994).
Fujimura-Kamada et al. (1997, J. Cell Biol. 136: 271-285) disclose a new subfamily of proteins that appear to function as intracellular, membrane-associated zinc metalloproteases. They disclose the Saccharomyces cerevisiae STE24 gene product which contains a zinc metalloprotease motif (HEXXH), as well as multiple predicted membrane spans. They further disclose that STE24 is required for the first NH2-terminal proteolytic cleavage event during biogenesis of the a-factor precursor.